A general FA system includes a master device, a slave device, and a device such as a sensor, and the master device controls operation of the device through the slave device and receives output data of the device. The slave device and the device are connected to each other by a signal line, and a digital output signal and an analog output signal are transmitted from the device to the slave device through the signal line.
In the case where abnormality such as a disconnection and a short circuit is generated in the signal line, it is necessary to detect the generation of the abnormality because the slave device cannot normally detect the output signal from the device.
For the signal line through which a digital signal is transmitted and received, a leakage current detector is mounted on the signal line in order to detect a leakage current, and the leakage current detector directly monitors a weak current passed during communication off of the digital signal, which allows the detection of the disconnection. A poly-switch in which a switch is turned off at a predetermined temperature or above is mounted to detect a temperature increase associated with short circuit of the signal line by the poly-switch, which allows the detection of the short circuit.
For example, Unexamined Japanese Patent Publication No. 8-285914 discloses a disconnection detecting circuit including such techniques. The disconnection detecting circuit disclosed in Unexamined Japanese Patent Publication No. 8-285914 includes a photocoupler that outputs generation of the disconnection of a lead (signal line). Specifically, in the case where the photocoupler does not detect the leakage current (in an OFF state), the generation of the disconnection is detected in the lead (signal line). Therefore, the disconnection of the lead (signal line) can be detected.
For the signal line through which an analog signal is transmitted and received, there is a technique of determining the generation of the disconnection when voltage applied to the signal line is smaller than a lower limit value of a measurement range of the analog signal.